Monomers which are curable with heat or an active energy line and used in coating compositions include monomers having a (meth) acryloyl group (acryloyl group or methacryloyl group) in the molecule, such as urethane (meth) acrylate, polyester (meth) acrylate and epoxy (meth) acrylate. Since urethane (meth) acrylate monomers out of these can be synthesized from a compound containing an isocyanate group and a compound containing a hydroxyl group or a carboxyl group, a wide variety of molecular designs are made possible by combining these compounds. Therefore, the urethane (meth) acrylate monomers are widely used as materials for coating agents, adhesives, coating compositions and plastic products.
Since the urethane (meth) acrylate monomers are synthesized from a compound containing an isocyanate group and a compound containing a hydroxyl group or a carboxyl group as described above, an acid component and a by-produced high-molecular weight impurity may remain in the obtained urethane (meth) acrylate monomers. This acid component may or may not cause a problem according to use purpose. For example, when a urethane (meth) acrylate monomer is used in a photosensitive resin, it does not cause a problem. For application in a photosensitive resin, a urethane (meth)acrylate monomer having an acid value of not less than 5 mgKOH/g is preferably used (refer to JP-A 2005-331932 and JP-A 2006-201546). Therefore, a large number of urethane (meth)acrylate monomers having a high acid value are available on the market.
However, the acid component contained in the urethane (meth)acrylate monomer may react with another monomer component or an additive component, thereby causing the coloration or discoloration of the obtained polymer in application fields other than photosensitive resins. A urethane (meth)acrylate monomer containing a large amount of an acid component and having a high acid value may deteriorate in storage stability, and there is room for improvement in this respect.
For example, for application in optical materials such as lenses, the requirements for a cured product include high strength and a small shrinkage factor. To meet these requirements, use of a urethane (meth)acrylate monomer as a component of a composition for the cured product is effective. However, the acid component contained in the urethane (meth)acrylate monomer causes the degradation of weather resistance. When the optical material is a photochromic lens material, the acid component may cause the deterioration of a photochromic dye. Therefore, there is also room for improvement in this respect.
Since the high-molecular weight impurity contained in the urethane (meth)acrylate monomer increases the viscosity of the monomer, it may make filtration operation at the time of production cumbersome and complicate. Further, this high-molecular weight impurity may become the cause of a deposit and may degrade the performance of the urethane (meth)acrylate monomer when the monomer is used in a primer.
To cope with these problems, there is known a method of reducing the amount of a high-molecular weight impurity by dissolving a urethane (meth)acrylate monomer containing a high-molecular weight impurity in a water-soluble organic solvent and contacting the obtained solution to an adsorbent such as activated carbon, a synthetic resin adsorbent or activated alumina (refer to JP-A 2007-63189). In this method, it is considered that particularly when activated alumina is used, an acid component can be also removed efficiently.
However, according to studies conducted by the inventors of the present invention, it was found that when activated alumina is used, there is a case where the high-molecular weight impurity cannot be removed completely though the acid component can be removed. It is disclosed in Examples of JP-A 2007-63189 that the content of a high-molecular weight impurity becomes 0.3% when a solution obtained by dissolving a urethane (meth)acrylate monomer containing 0.6% of a high-molecular weight impurity in ethanol is contacted to activated alumina. It was found that when a solution of a urethane (meth)acrylate monomer containing no high-molecular weight impurity is contacted to activated alumina, the activated alumina is filtered, and the filtrate is concentrated to produce urethane (meth) acrylate having a reduced acid value according to this method, about 0.3% of a high-molecular weight impurity is contained. This shows that a high-molecular weight impurity is newly produced when the urethane (meth)acrylate monomer is contacted to activated alumina as an adsorbent according to the method of JP-A 2007-63189.